Electrical utilities and principal power consuming industries have employed a variety of metering approaches to provide quantitative and qualitative evaluation of electrical power. The outputs provided by such metering systems vary to suit the particular needs of the user, selection of read-outs generally being made from the parameters including volthours, volt.sup.2 hours, watthours, Qhours, varhours, and VA hours. These quantities are designated as in or out depending upon the direction of current flow, the term "out" representing delivery to the user and the term "in" representing return of power to the generating entity.
Typically, a metering system monitors power supplied through isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs then are selectively treated to derive units of power and the like as above listed. The most extensively employed technique has been the measurement of watthours through the use of an electromechanical induction meter. However, such devices are limited and thus, there have developed electronic analog techniques for carrying out multiplication and phase adjustment to achieve higher accuracies and a multitude of readouts.
Early analog approaches taken to provide power parameter outputs initially involved the use of thermally responsive coil elements and the like, the temperatures of which could be converted to outputs corresponding with power values. A lack of convenience and accuracy with such techniques lead to interest in the utilization of Hall effect devices as multipliers wherein voltage-proportional generated magnetic field and current were associated to provide a voltage output proportional to the product of current and voltage. Other devices have been developed which utilize an electronic arrangement serving to capitalize on the exponential transfer characteristic of solid-state devices to carry out multiplication. In general, these early analog multiplication techniques were somewhat unsatisfactory in exhibiting inaccuracies lower than desired as well as instabilities.
Another analog multiplier technique currently popular in the industry utilizes the system concept of time division multiplication. For example, the multiplier produces a pulse waveform whose amplitude is proportional to one variable, whose length relative to period is a function of another variable, and whose average value is proportional to the product of the two values. A variety of improvements in such time division multiplier circuits have been developed with respect to controlling phase and phase derived inaccuracies. Such improvements, for example, are described in U.S. Pat. Nos. 4,356,446, issued October 26, 1982; and 4,408,283, issued October 4, 1983; both assigned to the Assignee of this invention.
Analog approaches to electrical parameter monitoring and multiplication techniques physically are beset with problems in achieving desired output accuracy. Accurate drift-free analog multipliers are somewhat expensive and generally exhibit undesirable drift and component variations from device to device. Accordingly, a considerable amount of technical effort is required in their production and maintenance to provide for adjustment for these various inadequacies. As a consequence of these deficiencies, other approaches have been contemplated by investigators. For example, should the line inputs be purely sinusoidal, then straightforward peak detection techniques associated with mathmatical evaluation would be available. However, the line inputs experienced worldwide, while basically resembling sinusoids, exhibit substantial variations representing high and low frequency noise, high energy transients and a multitude of variations. These variations generally are caused by any of a number of external phenomena, for example, rapidly changing loads developed from solid-state controllers such as silicon controlled rectifier driven devices. In effect, portions of the waveform may be essentially missing due to high speed switching at loads.
Purely digital approaches to measuring electric power have been contemplated as ideal. With such an arrangement, for example, high rates of sampling may be employed and the instantaneous sample values then may be converted or digitized as binary values. This ideal approach generally has been considered to require very high speed systems either unavailable or of such cost and complexity as to preclude utilization for the instant purpose. However, this idealized approach promises to avoid degradation of accuracy occuring due to component variations and drift phenomenon and environmental effects.
As a compromise to the above ideal high speed sampling, relatively slow sampling techniques, i.e. on the order of each 45.degree. of a cycle, have been proposed. To regain accuracy, such sampling is randomized. When using such randomized data, the approaches then must employ an averaging of the sampled values and thus, the advantages of high sample rate, instantaneous evaluation of waveform are not achieved nor can the systems distinguish discrete vagaries in distorted sinusoids. A typical randomizing approach is described, for example, in U.S. Pat. No. 4,077,061, issued February 28, 1978.
A further design aspect which has impeded the development of practical digital multiplication circuits resides in the somewhat limited range output of analog-to-digital conversion devices. Those available at practical cost, for example, provide a 12-bit output which generally will be found to be inadequate to achieve the scale of accuracy desired by industry. This particularly is true for those portions of a given sinusoid cycle which are of relatively lower amplitude as the cycle approaches cross-over. It is important that these lower level amplitudes be evaluated at high resolution accuracies for the approach to be practical. Some techniques for improving evaluation accuracies at lower amplitude have employed compressed scales to maximize resolution at lower levels. However, the full range bit resolution for such approaches remains unsatisfactory and complex and time demanding software overhead generally is consumed to accommodate to the compressed scaling.
Notwithstanding the foregoing, should a practical digital approach with high speed sampling be achieved, such system still must be capable of measuring all of the above-listed electrical parameters. Further, the technique must have reasonable accuracy such that from a system approach including all scaling components involving transformers, resistors and the like operation within an allowable error of .+-.0.09% of input, .+-.0.005% of rated input. Further, the multiplier electronics should be capable of performance within .+-.0.06% of input, .+-.0.005% of rated input. Thus, an allowable error of .+-.0.03% would be available for the input analog or scaling portion of any such device. Further, these systems should exhibit a reasonable dynamic operating range such as .+-.20% nominal voltage input, 0.025 to 200% of nominal current input and any power factor. Additionally, such system should be operable in conjunction with either single or polyphase power systems. This requires an approach involving a single phase metering technique such that single or polyphase calibration procedures may be employed. Thus, such system may not rely on the 120.degree. phase separation of three phase systems.